1. Field of the Invention
This invention relates to the nondestructive testing of gold pieces (bullion, bars coins . . . ) to ensure they contain only gold and have not been subject to dissimilar metal substitution.
2. Brief Description of the Related Art
Reports indicating that gold bullion is being physically hollowed and replaced with similar density metals, such as tungsten, are surfacing. While not a huge concern for the original manufacturer, the secondary exchange market can experience disruption if the integrity of the asset comes into question. Verifying the sample purity is critical and must be performed quickly, and nondestructively. Several potential test methods are applicable, but most are not good solutions for expense and technical reasons.
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies (or resonance frequencies) and are generally functions of the square root of the stiffness over the mass. Since each unique metal has a set of well defined elastic constants, the stiffness of the 2 metals will always be different, meaning that the resonances produced from any driving forces will be different and easily measured. Resonances can be produced from any swept sine spectrometer (preferred) or with an impulse function (striking with a hammer). Independent of how the resonance spectrum is created, it can be measured and compared with known good samples to observed whether or not it has been altered.
U.S. Pat. No. 5,922,956; “Dynamic Ultrasonic Resonant Testing, Rhodes, Jul. 13, 1999, describes a sample being excited by an exciting mechanical input (transducer) at a plurality of ultrasonic frequencies (the swept sine method), and sensing the resonant mechanical responses with the inverse process (1 or 2 mechanical receiving transducers). A dynamic signal analyzer is connected to receive the response of the sample and to output the resonance spectrum. A computer then determines the relevant resonances that adequately describe the conforming spectrum.
U.S. Pat. No. 5,495,763; Rhodes, et al. Mar. 5, 1996 entitled “Method for resonant measurement” first described the relevant resonance response characteristics of a sample being determined for use in characterizing the sample for non-destructive testing. Applying the same method to an adulterated sample, will immediately be obvious as the spectrum will shift according to the elastic property changes and some resonances will show additional differences in splitting and Q (the quality of the resonance as defined as the full width at half maximum), as was this is done by submitting numerous known conforming samples to examination and mapping the resonance responses. The conforming samples will produce a nearly identical spectrum, where nonconforming will show shifts, line splitting and Q differences.
In U.S. Pat. No. 5,062,296, Migliori described resonant ultrasound spectroscopy as a method to provide a unique characterization of an object for use in distinguishing similar objects having physical differences greater than a predetermined tolerance. A resonant response spectrum is obtained for a reference object by placing excitation and detection transducers at any accessible location on the object. The spectrum is analyzed to determine the number of resonant response peaks in a predetermined frequency interval. The distribution of the resonance frequencies is then characterized in a manner effective to form a unique signature of the object. In one characterization, a small frequency interval is defined and stepped though the spectrum frequency range. Subsequent objects are similarly characterized where the characterizations serve as signatures effective to distinguish objects that differ from the reference object by more than the predetermined tolerance.
U.S. Pat. Nos. 5,922,956, 5,495,763 and 5,062,296 are hereby incorporated by reference in their entirety.